How to Full Join Two Tables with Same Field Names in ETL

Scenario Description

In real business scenarios, business data is usually stored in multiple different tables, such as product tables, promotion tables, transaction tables, etc. When performing data analysis, it is often necessary to join multiple tables. For example, suppose we have the following two tables, the left and right tables, and need to perform a full join on field 'A'.

1. If you only select field 'A' from the left table, the result is:

2. If you only select field 'A' from the right table, the result is:

In these cases, the number of rows is correct, but the values are incomplete.

Cause

Although both tables have an 'A' field, they are actually two different fields: left.A and right.A. Their values are not fully inclusive. To get all values of 'A', we must keep both columns during the full join.

Solution Steps

1. Rename one of the 'A' fields, for example, rename the left table's 'A' to 'A1'.

2. Perform a full join on 'A1' and 'A', and select both 'A1' and 'A' columns as well as other needed columns.

3. In practice, we often want the following effect, which can be achieved using the COALESCE() function. The method is as follows:

1. Create a new column and use COALESCE(A1, A) to get all values of 'A'.

2. Then select columns and replace null values (if any).

3. If there are multiple fields with the same name in both tables, repeat this operation as needed.